I know HD has very little sheet steel in the store (more online but not big pieces). We used a heavy gauge of flashing (for flashing) that we found at ACE Hardware and then backed it. It was just barely wide enough to fit inside the stripped frames of the existing windows. Locally (Socorro), to buy sheet metal was incredibly high. No oil canning. No dings (not even after the hail storms). Easily cut. Just had to prime it with primer made for aluminum and galvanized metals. We will be doing another two windows with 28 ga galvanized steel (search for "flat flashing" or "metal flashing"). Two windows are trading places and then the two emergency windows will have the glass removed. The frames will be pulled out, cleaned up. The flashing will be held to the exterior side of the frames with 3M VHB Tape then the whole mess will be caulked into place. The metal will be primed and then painted to match the rest of the bus. On the inside, rigid foam sheathing will be glued into place (multiple layers of 1" thickness) and then covered over with click lock flooring (makes a good wall covering). Actually one of the plywood backed sheets is the only one with a messed up spot. But it is the only good way of securing vent covers. Because we used the window frames to support the metal then backed it with 1" rigid foam board (pink), it turned out pretty solid for such a small area. Not a whole lot different than what they do for a manufactured RV (they use a thin sheet of fiberglass not metal). We'll probably end up fiberglassing over plywood for the access hatches under the bus. Not a method for everyone but we did what we needed to do with what we could get within our limited budget.

DON"T buy at a hardware store. Go to a steel supplier, or even try a fabrication shop. I worked in plasma cutting and steel for many years and they are robbing you at Lowe's and HD.
Sheets can be ordered and cut for way less than one would think. Just call around. Could probably get a sheet and have it cut and done...
What thickness are you thinking? I'd go with something like 14 gauge.

DON"T buy at a hardware store. Go to a steel supplier, or even try a fabrication shop...

We tried that. EEK! Around $500 to skin over the 21 windows. I guess when you are the only game in town, you can charge anything you want. Our cost to skin over the windows from my spreadsheet "02/20/2010 ACE Hardware Aluminum Flashing 24”H x 50 ft Long $66.99" So $66.99 vs $500. Easy choice. This was the only place we ran into rolls of flashing that was 24". For the way we did our windows, it worked great.

Fiberglass.... That had me thinking.... Resin over plywood. A thin plywood glued to a thicker larger piece with the thin piece fitting the aperture and the larger behind the flange. Primed and painted over with a barrier paint then use some body filler to fill the join and paint the whole bus. Attach the plywood with double glazing glue.